/.file

This is an empty hidden file in the root of iOS. It appears to have no purpose, but it may be used for file integrity checks to verify the filesystem is not corrupt. According to Apple, .file can be used to make a “reference” to a file like: /.file/id=6571367.2773272, where the file has a composite key of device ID and inode ID; it is believed to be a fundamental part of how aliasing works.